How to move vertices closer together without actually scaling them
If I understand correctly, you want to move the selection of vertices in opposite directions on the X axis without deforming the scale, right? If so, do it like this ...


In Edit Mode, select the vertices.



In the Sidebar, select the Push / Pull tool.

In the Header, in Drag, choose Tweak to activate the tool's gizmo.

In the Header, in Transform Pivot Point, choose Individual Origins.




Then just drag the gizmo. The vertices will move without deforming the scale.